Obviously, something needs to give and that something is the staybolt. It bends.
Larger locomotives made it evident that even the most flexible material would not hold up in the extreme distances from the mud ring, and the flexible staybolt was developed to help cut down on breakage.
The subject of the staybolt became irrelevant and was dropped.
Welded staybolts mitigated the hardness differential between stay and sheet material, but the change also deleted the requirement for a key feature that monitors the staybolt.
That change was a very bad idea for steam locomotive boilers simply because the feature, known as a telltale, is the best indicator of when the staybolt breaks.
A telltale is a 3/16-inch diameter hole on the longitudinal axis of the staybolt. This hole can be drilled from each end a distance of no less than one-half inch beyond the water side of the sheet so as to include the highest stress area of the staybolt.
When a staybolt cracks, it typically cracks on one side only.
Square corners in fireboxes are easy to make but they impede circulation and often lead to a staybolt pitch problem on the wrapper sheets in the front and rear.
A key element that disappeared with the 1952 Code was the 7,500 psi stress limit on staybolts. This limit served the railroad industry well and was an acknowledgement that the stays did indeed bend and that minimizing the tensile stress tended to lengthen the staybolt's service life, especially when a stay broke between inspections and the stress was thrown on the adjacent staybolts.